1. The Field of the Invention
The present invention relates to edge emitting semiconductor lasers. More particularly, the present invention relates to single mode edge emitting semiconductor lasers using photonic crystals as mirrors.
3. Background and Relevant Art
Semiconductor lasers are often used as light sources in optical communication systems. The light emitted by a semiconductor laser is generated when photons stimulate the emission of additional photons in the gain medium or active region of the semiconductor laser. The gain medium is typically bounded by cleaved facets, which function as mirrors to reflect photons through the active region. The facets of the laser are usually parallel to each other and form a cavity that is called a Fabry-Perot cavity.
Optical amplification occurs in a Fabry-Perot cavity as light is. reflected back and forth between the facets through the gain medium. The cavity typically has resonant wavelengths, and for these resonant wavelengths, the reflected light adds in phase. The combination of adequate facet reflectivity and amplifier gain causes the laser to lase at resonant wavelengths. Although single mode behavior is desirable, many lasers often exhibit multimode behavior. Multimode is undesirable for applications such as communication systems that are modulated at high speeds. In particular, multimode behavior limits the usable bandwidth because of intensity noise and because different modes travel at different velocities in an optical fiber.
Single mode edge emitting lasers include distributed feedback (DFB) lasers and distributed Bragg reflector (DBR) lasers. DFB and DBR edge emitting lasers are created using a series of reflectors or a grating such as a Bragg reflector. The corrugation of the Bragg reflector can occur within and/or without the gain medium. In a DBR laser, the corrugation occurs outside of the gain medium. In a DFB laser, the corrugation occurs within the active region. The Bragg reflector or corrugation suppresses some of the modes of the laser while amplifying a particular mode.
Fabricating a DFB or DBR laser is usually performed using epitaxial regrowth, which is often expensive and can lead to decreased yield when the lasers are fabricated. In addition, DFB lasers are sensitive to where the facets are cleaved with respect to the grating or corrugation. Because the facet cleaving cannot be precisely controlled, the lasers fabricated in this manner can have variations in their performance. Because some of the lasers are discarded, the overall laser yield decreases and the cost per usable laser increases.
These and other limitations are overcome by the present invention which relates to light emitting sources, such as edge emitting lasers, that generate single mode output using photonic crystals as mirrors. As previously stated, Fabry-Perot lasers that emit light at multiple wavelengths or modes are problematic for various applications including long distance optical communication. The present invention achieves single mode behavior, for example, by using photonic crystals as mirrors in the edge emitting laser. In one embodiment, the gain cavity and/or the external/coupled cavity is defined by the photonic mirrors instead of cleaved facets. In accordance with the present invention, photonic mirrors are used in external or coupled cavity lasers and in short cavity lasers, or any combination thereof.
Photonic crystals or materials function as the mirrors of an edge emitting laser to reflect light through the laser cavities. Photonic crystals have a photonic bandgap and are reflective for certain wavelengths. These photonic mirrors are thus arranged such that a single mode is generated or emitted by the edge emitting laser. Exemplary embodiments of the present invention that include photonic crystals to emit a single mode include, but are not limited to, external or coupled cavity lasers and short cavity lasers.
In one embodiment, a single mode is emitted because the reflectivity of the photonic mirrors is dependent on the wavelength of the light. The photonic mirror regions of the edge emitting laser can be designed with respect to a particular wavelength such that the particular wavelength is reflected while other wavelengths are not reflected as well as the particular wavelength. The wavelengths that are not reflected do not have appreciable gain. The photonic mirrors ensure that wavelengths within the photonic band gap have appreciable gain.
In another embodiment, photonic crystals are used to form an external cavity or coupled cavity edge emitting laser. The photonic mirror regions are arranged to create a single mode emission from the edge emitting laser. In this example, the photonic mirror regions define a pair of cavities a gain cavity and an external or coupled cavity. In an external cavity embodiment, the second cavity does not have a gain medium. In a coupled cavity embodiment, both cavities may be pumped to generate laser light and the coupled cavity may include a gain medium.
In another embodiment of a coupled cavity or external cavity edge emitting laser, two photonic mirror regions may be used, while retaining single mode operation. In this example, the external or coupled cavity is bounded by the photonic mirrors while the gain cavity is bounded by a cleaved facet and a photonic mirror. Alternatively, the gain cavity is bounded by the photonic mirrors while the external cavity is bounded by one of the photonic mirrors and a cleaved facet. In another embodiment, a single photonic mirror is used to separate the gain cavity and the coupled or external cavity.
In yet another embodiment, photonic crystals can be used to create short cavity lasers. Typically, short cavity lasers are difficult to fabricate because the location of the cleaved facet is difficult to control. Fabricating short cavity lasers is also difficult because semiconductor materials are rather brittle and hard to cut, which presents mechanical difficulties when cleaving a laser. Photonic crystals can be more precisely placed such that a short cavity edge emitting laser is formed.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.